This invention relates to a process for making an elastically stretchable composite sheet and more particularly such a composite sheet offering a comfortable touch being suitable as an important component of disposable garments such as disposable diapers, sanitary napkins and disposable gowns used in medical site.
Japanese Patent Publication No. 1995-37703 discloses a process for making a nonwoven elastic sheet comprising the steps of continuously feeding elastic web with a tension in a machine direction, placing a fibrous web on the upper surface of the elastic web, bonding them together by heat- or ultrasonic-sealing and relaxing the elastic web to contract so that the fibrous web may form gathers. The nonwoven elastic sheet thus obtained has an elasticity generated by the presence of the elastic web and a comfortably soft touch so that such nonwoven elastic sheet may be suitably used as cover material of disposable diapers or sanitary napkins.
According to the process of prior art, the elastic web is relieved to contract first after the fibrous web has been bonded with the elastic web still under tension. With a consequence, the process of prior art is necessarily accompanied with a problem that a basis weight of the fibrous web being fed inevitably increases as the elastic web contracts after the fibrous web has been bonded with the elastic web.
This invention aims to provide a process for making an elastically stretchable composite sheet enabling the fibrous web in the finished composite sheet to maintain substantially the same basis weight as that of the fibrous web being fed and thereby to solve the problem in the process of prior art.
According to this invention, there is provided a process for making a composite sheet comprising a step of bonding a first web made of thermoplastic synthetic fibers and having an inelastic stretchability in one direction to at least one surface of a second web made of thermoplastic synthetic fibers having an elastic stretchability at least in the one direction and thereby to obtain the composite sheet having an elastic stretchability in said one direction, wherein:
the first web is made of stretchable synthetic continuous fibers having a breaking extension at least of 70% while the second web has its breaking extension higher than that of the first web and these first and second webs are bonded in accordance with the steps of:
a. continuously feeding the first web in the one direction;
b. continuously feeding the second web in the one direction so as to be placed upon the first web;
c. bonding the first and second webs placed upon each other together at first bond regions arranged intermittently at least along the one direction rather than in a direction which is orthogonal to the one direction;
d. stretching the first and second webs bonded together at least in the one direction rather than the direction which is orthogonal to the one direction within a critical elasticity of the second web and a critical breaking extension of the first web; and
e. elastically relaxing the stretched first and second webs to contract and then supplementarily bonding the first and second webs together at second bond regions arranged intermittently at least the one direction rather than the direction being orthogonal to the one direction and having a total area larger than a total area of the first bond regions to obtain the composite sheet.